Width and Height NSLayoutConstraints on UIImageView not working

I am setting the .Width and .Top NSLayoutConstraints on a UIImgeView instance inside my View Controller's

updateViewConstraints()

. In the XCode visual debugger i can see the constraints are actually set, but for some reason they are not being used (?) they appear un (content size) and are greyed out.
When i output the constraints using NSLog i can also see they are set, but somehow the image stays the original size ...
What does this mean?

This is how the layout constraints are added initially using IB. As you can see they are added for the Compact Width size Class, which i am using when i see the problem (iPhone4S simulator).

The constraints are already added via IB and i am updating their constant with code. The debugger shows clearly that the constant IS IN FACT UPDATED but somehow the image gets drawn in the original size.